OFFSET
1,2
REFERENCES
J.-M. De Koninck and A. Mercier, 1001 Problemes en Theorie Classique Des Nombres, Problem 749, pp. 95, 319, Ellipses, Paris, 2004.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..143
EXAMPLE
For example: 864 is a term since phi(864) = 288, sigma(864) = 2520, 864 = 2^5*3^3, (288+2520)/6^2 = 78.
MATHEMATICA
f[n_] := (DivisorSigma[1, n] + EulerPhi[n])/(Times @@ Transpose[FactorInteger[n]][[1]])^2; Do[ If[IntegerQ[f[n] && f[n] != 1], Print[n]], {n, 1, 1000000}] (* Tanya Khovanova, Aug 30 2006 *)
f1[p_, e_] := (p^(e + 1) - 1)/(p - 1); f2[p_, e_] := (p - 1)*p^(e - 1); q[1] = True; q[n_] := IntegerQ[(r = (Times @@ f1 @@@ (f = FactorInteger[n]) + Times @@ f2 @@@ f)/ (Times @@ First /@ f)^2)] && r > 1; Select[Range[10^5], q] (* Amiram Eldar, Dec 04 2020 *)
PROG
(PARI) rad(n)=my(f=factor(n)[, 1]); prod(i=1, #f, f[i])
is(n)=my(t=(eulerphi(n)+sigma(n))/rad(n)^2); denominator(t)==1 && t>1 \\ Charles R Greathouse IV, Feb 19 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Lekraj Beedassy, Sep 07 2004
EXTENSIONS
More terms from Tanya Khovanova, Aug 30 2006
a(15)-a(29) from Donovan Johnson, Feb 05 2010
a(1)=1 and a(30)-a(32) added by Amiram Eldar, Dec 04 2020
STATUS
approved